1. Field of the Invention
The present invention relates to a power generating system using a fuel cell apparatus and an operating method thereof. More particularly, the present invention relates to an art which improves the cooling method of a fuel cell apparatus, thereby simplifying a power generating system, improving reliability, and permitting operation with a high power density.
2. Description of the Related Art
FIG. 5 is a system configuration diagram illustrating an outline, in a fuel cell power generating system disclosed in Japanese Unexamined Patent Publication No. H04-79,166, of a fuel cell apparatus and a peripheral apparatus of temperature control of this fuel cell apparatus. In FIG. 5, (1) is a fuel cell apparatus, and (2) is a fuel cell section of a laminated structure representing a main portion of the fuel cell apparatus (1), which comprises a fuel cell unit (3) having a fuel gas electrode and an oxidizing gas electrode (not shown), a fuel gas channel (4) for supplying a fuel gas (A) to the fuel gas electrode, and an oxidizing gas channel (5) for supplying an oxidizing gas (B) to the oxidizing gas electrode, as main components. Also in FIG. 5, (6) is a reforming reaction section which is thermally combined with the fuel cell section (2) by arranging the reforming reaction section adjacent the fuel cell section (2). In an example, the fuel cell apparatus (1) has a structure in which flat sheet-shaped fuel cell sections (2) and flat sheet-shaped reforming reaction sections (6) are alternately laminated.
In FIG. 5, (7) is an air feeder which recovers power from waste gas (C) discharged from the fuel cell power generating system and supplies air (D) from outside by increasing pressure, and (8) is a circulating blower which circulates the reformed gas containing a hydrocarbon for temperature control of the fuel cell apparatus (1). In the same drawing, (9) is a methanator which methanates hydrogen, carbon monoxide and carbon dioxide contained in the reformed gas circulated by means of the circulating blower (8).
Now, operation of this conventional case will be described below. During constant-load operation of the fuel cell apparatus (1), a waste heat generated, corresponding for example to 30 to 70% of output power, must be efficiently eliminated. In the conventional case shown in FIG. 5, the fuel cell section (2) is cooled by introducing the reforming gas containing a hydrocarbon or an alcohol and steam into the reforming reaction section (6) holding a reforming catalyst in the inside thereof, and causing a reforming reaction which is an endothermic reaction in this reforming reaction section (6). In this example, the fuel cell apparatus (1) has a laminated structure in which, for example, a plurality of fuel cell sections (2) each comprising several laminated flat sheet-shaped fuel cells and flat sheet-shaped reforming reaction sections (6) are alternately laminated. When the fuel cell is a molten carbonate type one, reforming reactions expressed by the following formulae (1) to (3) proceed to the right at a temperature within a general operating temperature of from 600 to 700.degree. C. in the reforming reaction sections (6): EQU C.sub.n H.sub.m +nH.sub.2 O.fwdarw.nCO+(2n+m)/2*H.sub.2 (1) EQU Alcohol+H.sub.2 O.fwdarw.CO, CO.sub.2, H.sub.2 (2) EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 (3)
The reformed gas containing hydrogen, carbon monoxide and carbon dioxide produced by the reforming reactions expressed by the foregoing formulae (1) to (3) as main constituents is supplied to a methanator (9) by the action of a circulating blower (8). The methanator (9) is usually a heat-exchanger type reactor having therein a methanation catalyst, and comprises a reaction-side space holding the methanation catalyst in a state capable of coming into contact with the reformed gas (F), and a cooling-side space having the heat-exchanging relationship with the reaction-side space and through which a coolant (G) flows. In this conventional case, the methanator (9) is operated at an operating temperature (for example, about 250 to 500.degree. C.) lower than the operating temperature (600 to 700.degree. C.) of the reforming reaction section (6).
The reforming reaction expressed by the formula (3) is a reversible reaction. The methane producing reaction (methanation reaction) toward the left in the formula (3) proceeds more according as the operating temperature becomes lower, and at the same time, tends to discharge a reaction heat. More specifically, the reaction heat produced along with the cell reaction in the fuel cell section (2) is discharged to outside the fuel cell apparatus (1) through progress of the reforming reaction (an endothermic reaction) of hydrocarbon (methane) in the reaction gas by the action of the reforming reaction section (6). The waste heat of the fuel cell discharged outside is discharged into the coolant (G) flowing on the cooling side of the methanator (9) along with the progress of the methanation reaction (an-exothermic reaction) of the reaction gas (F) in the methanator (9).
In the foregoing conventional case, as described above, the reformed gas (F) is caused to circulate between the reforming reaction section (6) and the methanator (9). An endothermic reaction proceeds in the reforming reaction section (6), and the fuel cell section (2) is cooled by causing an exothermic reaction in the methanator (9).
When cooling a molten carbonate type fuel cell stack operating at a high temperature of the order of from 600 to 700.degree. C., it is difficult to apply liquid cooling or ebullition cooling which is an efficient cooling method, and therefore, it has been the conventional common practice to apply gas cooling using a reaction gas such as an oxidizing gas. Gas cooling has however posed problems in terms of the efficiency of the power generating system, performance of the fuel cell apparatus and maneuverability such as a large temperature distribution (for example, about 100.degree. C.) produced in the flow direction of the cooling gas, necessity of a large auxiliary power for circulating the cooling gas, a large amount of oxidizing gas flowing through the oxidizing gas channel adjacent to the oxidizing gas electrode, resulting in a large pressure loss, increase in the pressure difference between the fuel gas side and the oxidizing gas in the fuel cell, leading to a higher risk of cross leakage, and necessity of an oxidizing gas channel having a large sectional area. These defects have been particularly serious when operating a fell cell continuously at a higher power density, i.e., with the fuel cell in a state of a high current density (for example, current density: 200 to 300 mA/cm.sup.2.
The conventional case shown in FIG. 5 was conducted to solve the problems as described above. Because of the cooling system based on reforming reaction heat in the conventional case shown in FIG. 5, the takeout amount of heat per unit quantity of gas is 100 to 150 times as large as that in a simple gas cooling, and it was possible to cool the stack with a small cooling gas flow rate. Furthermore, since cooling is based on reaction heat of reformed gas unlike in the gas cooling based on sensible heat, it was possible to accomplish cooling at a uniform temperature throughout the entire surface of the fuel cell in principle, and even in a high current-density operation, to ensure stable operation with a uniform temperature distribution free from such problems as creation of a hot spot.
However, in the conventional power generating system having the configuration as described above, it is necessary to ensure circulation of the reformed gas which is a high-temperature combustible wet gas through a circulation channel via a circulating blower (8) as a coolant for cooling the fuel cell stack. It is therefore difficult to provide a good gas sealing at the driving shaft of the circulating blower (8), resulting in technical problems in practice including the necessity of an expensive circulating blower (8). An auxiliary power is still required for circulation of the reformed gas. Furthermore, a piping for the reformed gas which serves for cooling is necessary, in addition to the piping for the fuel gas and the oxidizing gas participating in the cell reactions, as a takeout from the fuel cell. This poses another problem of an increased number of piping systems and the increased amount of radiant heat from the piping.
Apart from the necessity of an expensive high-temperature circulating equipment and a high-temperature piping for building a cooling system, an auxiliary power is required, and there is an increase in the amount of radiant heat from the piping, leading to problems in cost, power generating efficiency and reliability of the power generating system.
The present invention was developed to solve these problems as described above, and has an object to provide a low-cost fuel cell power generating system high in reliability and power generating efficiency and excellent in maneuverability, which eliminates the necessity of a fuel gas circulating blower or an excessive piping for the fuel gas for cooling.